This invention relates to a method and device for the precise geometrical positioning of work pieces having irregular or random surface configurations which are to be machined.
When blanks are to be machined by a machine tool in the manufacture of finished work pieces, such as parts for machines or for aircraft, the blanks must be correctly and precisely positioned in space relative to a three-dimensional coordinate system as determined by the machine tool so that the parts are properly accessible to the machining tools. The work piece may be positioned with respect to one, two or three planes in a three-dimensional coordinate system having the space coordinates X, Y, Z.
According to the invention the work piece is positioned by three positioning devices before any clamping of the work piece. A clear distinction must be made between positioning, supporting and clamping of a work piece because otherwise it is not possible to properly understand this invention. The positioning of a work piece must take place first for defining its location in a fixed three-dimensional coordinate system or at least relative to one defined plane in such a fixed coordinate system. Precision machining cannot be performed without precision locating of the work piece. The supporting of the work piece in the machine tool must take place after the positioning. The supporting takes up the weight of the work piece and must not under any circumstances disturb the already established precise position of the work piece. Theoretically the supporting of the work piece may involve a suspending of the work piece so that the three positioning devices would not participate in bearing the weight of the work piece. The clamping of the work piece, which also takes place after the positioning must again not disturb the precise work piece position.
The positioning with respect to a first plane in a rectangular three-dimensional coordinate system must be highly accurate and has been very time-consuming heretofore especially when the blanks have uneven or random surfaces. This positioning with respect to the first plane often could not be effected with the required accuracy heretofore unless the surface of the blank to be engaged by the machine tool members has been machined prior to the positioning or seating. The invention avoids such preliminary machining.
During the clamping of the positioned blank, the presence of uneven surfaces of the blank often gave rise to an undesired straining of the blank so that the machining would result in a defective work piece because after release of the machined work piece the latter could flex back into a shape it had prior to its clamping thereby preventing a precision machining. Even when the engaged surfaces of the blank have been machined prior to a conventional clamping, it is not always ensured that the work piece will not be strained and elastically deformed as it is clamped. Previously these difficulties have been kept within tolerable limits in that the surfaces for engaging the work piece for the positioning with respect to the first plane were made as accurate as necessary by a preliminary machining operation although this required a high additional labor expenditure. Thus, avoiding a preliminary machining operation would constitute a substantial advance in the art.
From U.S. Pat. No. 2,325,387 (Fredrickson) it is known that work pieces to be machined can be clamped by a clamping bar which carries two spherical elements, each of which has a base for engaging the work piece whereas each spherical surface is fitted in a seat formed in the clamping bar which itself is pivotable for accommodating different work pieces which differ in height. In accordance with said references, the work piece is positioned and clamped in the usual manner simultaneously whereby the clamping can defeat the desired precision positioning of the work piece.
U.S. Pat. No. 1,788,652 (Andrew et al) shows a work piece clamping device in which machined surfaces are required for the clamping by two lateral clamps and by two end clamps. Each of the end clamps has a clamping member with a spherical surface portion held in a spherical recess for permitting slight variations in the position of the clamp relative to the work piece. Andrew et al does not make use of the fact that three and only three points, the coordinates of which are given, determine geometrically a plane in a coordinate system. Andrew et al's clamping member with the spherical surface does not directly contact the work piece, it merely permits slight adjustments of the clamping elements.
East German Pat. No. 121,295 (Dassler et al) supports and simultaneously clamps a work piece at three locations whereby each of the three locations is provided with three support cams, providing a total of nine support cams which defeats the effective utilization of three and only three position determining points. The work piece is supported at best, but not position determined in such a way that the clamping will be prevented from disturbing the work piece position.